To Purge, or not to Purge

I was discussing a recent study in Bozeman with a student of groundwater hydrology where the local water quality district tested values of nitrates from wells that are purged before sampling versus nitrate levels from wells that were not purged. The results indicated that the difference in values between purged wells and water that had been sitting stagnant in the casing was subtle, but apparent.

I have not been able to find the study, as I don't think the results have been published, but this is an interesting topic since most of the groundwater in privately-owned wells is tested by untrained well owners who do not purge their wells. Thus, while the difference is not enough to be significant for owners who are just finding a baseline value for their drinking water, it is significant in terms of the data collected from owners throughout the state which is used to examine trends in groundwater by, say, the university.

When I worked for the USGS in California, we always sampled from purged wells to ensure that we sampled a representative portion of the groundwater, and not just stagnant water that hung around in the casing and did not represent the composition of the groundwater. We thus ensured an unbiased sample and that the results of our lab tests accurately reflected the actual groundwater below the well.

Nitrates

Nitrate (NO3) is a naturally-occuring compound made up of nitrogen and oxygen that is an essential nutrient for plants. It forms when ammonia or other sources of nitrogen combine with oxygen in water. Nitrate is found in plants, soil, animal manure, septic systems, sewage, fertilizers, and airborne components from industry that are deposited through precipitation or dry particles.

Nitrate is highly soluble in water and does not easily bind with other compounds, making it more likely to remain in the water cycle and thus move large distances and remain in solution in rivers, streams, or groundwater for long periods of time. This means that one could find nitrate in high concentrations miles from the original source, and years after the original source was gone.

Infants under 6 months are susceptible to methemoglobinemia, where the nitrates are converted to nitrites in their stomachs (due to bacteria in their stomachs) which then enter the bloodstream and subsequently interferes with the body's ability to carry oxygen to tissues. In children and adults, nitrates may be linked to birth defects and certain types of cancer.

So what does this mean? Long-term exposure to high levels of nitrate poses a health hazard, and therefore well owners should regularly test their groundwater for levels of nitrate above the MCL (maximum contaminant level), which is 10 mg/L for both Montana and California.

Image: American Falls at Idaho Falls, ID (July 2008)